(1) Field of the Invention
The present invention relates to an image forming apparatus such as a page printer, a copying machine or the like. More specifically, to an image forming apparatus designed to form a multi-image by irradiating several laser beams or the like onto a photosensitive body.
(2) Description of the Related Art
A conventional image forming apparatus shown in FIG. 7 is designed to form a multi-image in two or more colors by developing an electrostatic latent image by a developing device 3a after the electrostatic lament image is formed on the surface of a photosensitive drum 30 by irradiating an optical beam B1, and subsequently, by developing another electrostatic latent image by a developing device 3b after the electrostatic latent image is formed on the surface of the photosensitive drum 30 which has passed the developing device 3a.
In a conventional image forming apparatus, irradiating position of the optical beams B1 and B2, timings of irradiation of the optical beams B1 and B2, and a speed of rotation of the photosensitive drum 30 should maintain a certain correlation, otherwise the image formed by the optical beam B1 and B2 respectively will not be formed in a specified position which results in unexpected color positioning and the deterioration of the image quality.
Japanese utility model Kokai No. 63-35052 teaches an image forming apparatus provided with an optical sensor at the lower irradiating position of the optical beam in the rotating direction of the photosensitive body, wherein a timing of the irradiation of a lower optical beam is controlled by the optical sensor in accordance with the timing when the optical sensor detects a starting position from the image formed by the upper optical beam.
However, in the above apparatus, displacement between the two images continuously occurs until the irradiating position of the lower optical beam strictly fits to the position detected by the optical sensor. Therefore, a great amount of time and labor has been consumed to fit the irradiating position to the position detected by the optical sensor.